Student attendance in the Issaquah School District tumbled Thursday, and absences spiked by 70 percent, as police investigated a shooting threat against Skyline High School.

District administrators decided late Wednesday to close Skyline on Thursday and keep other campuses open. Staffers and students at other schools felt the ripples early.

Districtwide, absences increased at 18 of 25 schools Thursday, although not every instance is attributable to the Skyline threat.

Officials counted 1,158 students absent Thursday, up from 680 a week earlier, Sept. 13, according to a comparison of attendance data. The district serves about 17,000 students from Preston to Newcastle, and from Sammamish to Renton.

The sharpest increases in absences occurred at Beaver Lake Middle School (320 percent — or 105 absences, up from 25 a week earlier), Pine Lake Middle School (204 percent — or 137 absences, up from 44 a week earlier), Creekside Elementary School (163 percent — or 50 absences, up from 28 a week earlier), Sunny Hills Elementary School (154 percent — or 33 absences, up from 15 a week earlier), and Pacific Cascade Middle School (145 percent — or 49 absences, up from 20 a week earlier).

The largest increases in absences occurred at other Sammamish Plateau schools not far from Skyline.

Issaquah High School recorded the most absences overall Thursday — 203 students did not go to school. Compared to attendance data from a week earlier, absences jumped 66 percent.

Liberty High School, 13 miles southwest of Skyline, experienced a 60 percent increase in absences from a week earlier. Overall, 155 students did not go to school Thursday.

“If there was ever a day when people were feeling uncomfortable about coming to school or safety was an issue, it certainly could have been today,” district spokeswoman Sara Niegowski said late Thursday afternoon.

Early Thursday morning, administrators sent information to assuage parents about the threat, and noted “all schools have flexibility around absences during extraordinary circumstances.”

“I have no way to say there is a direct correlation, but I would strongly believe that it had to do with the disturbance at Skyline High School,” Niegowski said. “Even though there was no threat, or we had no reason to believe that any other school was in danger, I think that it got into people’s psyche. It scared them. It scared all of us.”

Skyline is due to reopen Friday, with more police on campus, and opportunities for students and parents to meet counselors to discuss the incident. (Skyline claims almost 2,000 students between ninth and 12th grades.)

Eastside Catholic High School — a parochial school less than a mile north of Skyline along 228th Avenue — is closed Friday as a precautionary measure. Sister Mary Tracy, Eastside Catholic president, could not be reached for comment Thursday afternoon.

Cleveland-based school safety consultant Kenneth Trump said school administrators face a double-edged sword by closing school in response to a threat. (Trump, president of National School Safety and Security Services, is a longtime expert on school safety and has advised educators in Washington state.)

“How long do you keep school closed? That’s the very tricky part,” he said. “First of all, if you closed the school every time you received a threat, you’d be receiving a threat every day. At the same time, all threats have to be treated seriously and investigated. School and safety officials can’t summarily dismiss any single threat. The challenge is to strike a balance there.”

Trump said a school closure longer than a single day can cause parents’ belief in the district to wane.

“What happens after the first day is, parents will understand,” he continued. “But if you’re closed the second, third and fourth day, eventually parents lose confidence in their school and public safety officials, and start questioning whether they have control of the situation.”

The response started late Wednesday after Issaquah school officials discovered a mass shooting planned for Thursday at Skyline.

Police continue to follow several leads on the identity of the user behind the threat posted on the online bulletin board 4chan. Investigators have not yet been able to determine if the user is a Skyline student.

In the post, the user outlined plans to take his or her father’s Erma sub-machine gun and “open fire on the people in the commons” until law enforcement officers killed the shooter, or until he or she ran out of ammunition.

The post also included a photo purporting to show the sub-machine gun.

In a news conference at Sammamish City Hall late Thursday afternoon, King County Sheriff Steve Strachan said the photo used in the post is similar to stock art and is familiar to law enforcement.

Early Thursday afternoon, Sammamish Police Administrative Sgt. Jessica Sullivan said the FBI had traced the post to a proxy server in Sweden — a common technique among computer users seeking to mask their identities.

Families of Skyline students received emails and phone calls late Wednesday with information about the closure.

Trump lauded the Issaquah School District for using available resources — Facebook and Twitter, in addition to more traditional email and phone messages — to alert the school community.